1. Field of the Invention
This invention relates to an apparatus for scanning acoustic waves.
2. Description of the Prior Art
Acoustic wave scanning is accomplished by causing a acoustic beam always to be around an presumed axis moving on a certain surface which is plane or curved, or by detecting acoustic waves at each point on such axis. Here, the presumed axis means a predetermined direction in which it is expected to obtain information of an object. The movement of the presumed axis may be rotational or translational. Usually, the former is called the sector scan and the latter the linear scan.
The acoustic wave scanning method is classified into the type which comprises rotating or translating a single transducer and the type which uses a transducer element array. The present invention relates to the latter type of method. Therefore, the method using a transducer element array will now be described. For example, the simplest scanning method using a one-dimensional element array is the scanning by successive switching over the array elements. This method, however, has required individual transducer elements to be larger in size in order to reduce the divergence of the acoustic beam. The larger size of the elements has led to greater intervals between acoustic beams, which in turn has led to the disadvantage of a reduced resolving power. To avoid this, there has come into practice a method which comprises subdividing a relatively large transducer element into a plurality of smaller transducer elements and successively displacing the plurality of smaller transducer elements laterally by an amount corresponding to the width a smaller element, thereby effecting the scanning. According to this method, the divergence of the acoustic beam is small as in the previously described method, and the interval between acoustic beams is narrow, so that the resolving power is higher than in the previously described method. Nevertheless, the cross-sectional area of the acoustic beam is large and therefore, this method cannot be adopted where a high resolving power is required. As a remedy to this, there is known the so-called electronic focusing method which comprises imparting an operation time difference distribution to a plurality of the subdivided transducer elements to thereby focus the acoustic wave from said plurality of elements on a point on the presumed axis. This electronic focusing method is highly effective only for the focused point, and on the other portions of the presumed axis the acoustic wave is not focused. Therefore, the resolving power in these portions is not satisfactory.